U–Pb zircon geochronology and geochemistry of the Motaghairat intrusion, South Eastern Desert, Egypt: exposed mafic–ultramafic roots of a Neoproterozoic oceanic island arc

The Motaghairat layered mafic–ultramafic intrusion is part of the Neoproterozoic basement of the Eastern Desert of Egypt, constituting the northern tip of the juvenile Arabian-Nubian Shield (ANS). This intrusion exhibits a well-differentiated rock association, comprising, from bottom to top, dunite, harzburgite, lherzolite, orthopyroxenite, troctolite, olivine gabbro, and gabbro. Textural features, mineral compositions of olivine, pyroxene, and Cr-spinel, along with whole-rock geochemical data, collectively suggest that these rocks represent cumulates differentiated from a tholeiitic magma, essentially by fractional crystallization. The trace element patterns of pyroxenes, amphiboles, whole-rock samples, and calculated melts in equilibrium with clinopyroxene and amphibole exhibit pronounced Nb–Ta and Zr–Hf troughs, marked Pb peak, and enrichment in LILE coupled with HFSE depletion in the melts patterns, all of which are indicative of magma generation in a subduction-related arc setting from depleted mantle source metasomatized by slab-derived aqueous fluids. Moreover, zircon trace element compositions further support an oceanic island arc setting, displaying geochemical signatures akin to zircons from modern island arc magmas. The calculated ƒO2 values (FMQ +1 to FMQ + 3.7) and high water content of the parental magma (3.6–9 wt.%) confirm crystallization under oxidizing and hydrous conditions. The estimated pressure (~280 to ~550MPa) reflects crystallization depths from ~9 km to ~16 km within the oceanic arc crust. The age determined for the Motaghairat intrusion (715–703 Ma) for the first time in the present work provides new evidence that Neoproterozoic oceanic arc magmatism in the Mozambique Ocean continued to at least 700 Ma. Thus, the geochemical, mineralogical, and geochronological results of the present work collectively suggest that the Motaghairat layered mafic–ultramafic intrusion represents rare exposed deep ultramafic to gabbroic roots of the Neoproterozoic oceanic island arc.